The present invention is directed to probe stations suitable for making low current and low voltage measurements and, more particularly, to a system for reducing noise due to capacitive currents resulting from the operation of a thermal chuck for a probe station.
Integrated circuit devices are typically manufactured in and on a wafer of semiconductor material using well-known techniques. Prior to cutting the individual integrated circuit devices from a wafer, tests are run on individual devices to determine if the devices operate properly. The wafer is supported on a chuck inside an environmental enclosure in a probe station. Probes are brought into contact with test points or pads on the integrated circuit devices and a series of measurements are preformed. Schwindt et al., U.S. Pat. No. 5,663,653, disclose an example of a probe station in which the present invention might be used and the patent is incorporated herein by reference.
Many integrated circuit devices are designed to operate at temperatures other than room temperature. To accommodate device testing at temperatures other than the ambient temperature, a thermal chuck may be employed. One design of a thermal chuck comprises a multilayered chuck for securing a wafer having a thermal driver to modify the temperature of the chuck. A thermal chuck of this design is disclosed by Schwindt in U.S. Pat. No. 5,610,529 which is incorporated herein by reference.
The thermal driver may provide for either heating, cooling, or heating and cooling of the chuck. To modify the temperature of the chuck, the thermal driver may comprise one or more thermal units including a thermal device and a plurality of power conductors connecting the thermal device to a power source. Thermal devices, typically electric resistance heaters or thermoelectric heat pumps, are provided to heat the chuck to temperatures above the ambient temperature. The thermoelectric heat pump, also known as a Peltier device, is reversible and can be used for cooling as well as heating the chuck. The thermoelectric heat pump comprises a number of thermocouples sandwiched between two electrically insulating, thermally conductive plates. When DC power is supplied to the thermocouples, the Peltier effect causes heat to be transferred from one plate to the other. The direction of heat flow is reversible by reversing the direction of current flow in the thermocouples. Exposing the chuck to the warmer plate or the cooler plate of the thermoelectric heat pump will, respectively, either heat or cool the chuck. For testing at temperatures below ambient, the thermal chuck may also include passages for circulating coolant to cool the chuck directly or remove excess heat from the thermoelectric heat pump.
When making the low voltage and low current measurements common to testing integrated circuit devices, even very low levels of electrical noise are unsatisfactory. Thermal chucks include several sources of noise and unacceptably high levels of noise are a common problem when using a thermal chuck. One known source of noise is the result of expansion or contraction of the components of the thermal chuck due to changing temperature. Expansion or contraction changes the spacing between conductive components resulting in the generation of capacitive currents which can reach the conductive surface of the chuck. Expansion or contraction due to temperature change can also cause relative transverse movement between the multiple material layers of the chuck. Relative movement between contacting layers of insulating and conductive materials can generate triboelectric current. In a probe station chuck, the triboelectric current can appear as noise in the test measurements. Triboelectric currents can be reduced by a chuck design which prevents movement between contacting layers of insulating and conducting materials.
The operation of the thermal units by the thermal driver controller is another potential source of noise when using a thermal chuck. To change or maintain the temperature of the thermal chuck, the thermal driver controller fluctuates the electrical power to the thermal units in response to a temperature control system. As a result of the voltage drop within the conductors of the thermal units, physically adjacent portions of the electrical conductors leading to and from, and internal to the thermal devices, will be at different potentials. As the power fluctuates, the difference in voltage between the power conductors changes with time. This results in a displacement of charges in the dielectric material surrounding the conductors which manifests itself as a displacement or capacitive current coupled to the conductive top surface of the chuck. This capacitive current appears as noise in the test measurements.
The currently accepted technique to reduce the effects of capacitive currents involves shielding the chuck from external electromagnetic sources. However, the shielding layers of conductive material in the chuck have proven unsuccessful in eliminating the noise from the thermal driver. To reduce noise due to capacitive currents originating in the thermal chuck, users of probe stations often shut off the thermal units and wait for the current to dissipate. However, the RC time constant involved can be greater than five seconds. Waiting a period of five time constants (e.g. 25 seconds) for the observed noise to dissipate to an acceptable level before making a measurement substantially effects the productivity of the probe station.
What is desired, therefore, is a system for reducing the electrical noise generated by the operation of the thermal unit of a probe station""s thermal chuck. Reducing noise generated by the thermal chuck reduces the time for the noise to dissipate to acceptable levels improving the productivity of the probe station.
The present invention overcomes the aforementioned drawbacks of the prior art by providing a thermal chuck for a probe station comprising a chuck for supporting a device under test; a thermal unit for modifying the temperature of the chuck; a controller that supplies electrical power to the thermal unit; and a conductive member capacitively coupled to the thermal unit while being free from direct electrical connection to the thermal unit and being electrically interconnected to the controller so as to provide a conductive return path to the controller for substantially all of the capacitive currents emanating from the thermal unit during operation. The conductive member intercepts the capacitive currents emanating from the thermal units of the thermal driver and conductively returns them to the thermal driver controller located outside of the enclosure.
In a preferred embodiment of the invention a conductive extension of the grounded, conductive enclosure, within which the chuck is located, is capacitively connected to the thermal unit while being free from direct electrical connection thereto and while being electrically interconnected to ground so as to provide a conductive path to ground for capacitive currents emanating from the thermal unit. In this embodiment, the extension of the conductive enclosure intercepts capacitive currents which might emanate from portions of the thermal unit and which might leak from other capacitively coupled shielding of the thermal unit. As a result of inclusion of capacitively coupled shielding of the thermal unit, noise in the test measurements due to the operation of the thermal units is eliminated or substantially reduced. The productivity of the probe station can be increased because less time is required for noise originating from the operation of the thermal units to dissipate to acceptable levels.
The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.